Generally, light-emitting modules using LEDs are manufactured by arranging a plurality of LEDs on a surface of a resin substrate in a proper alignment, electrically connecting adjacent LEDs via a bonding wire, and sealing the LEDs on the surface of the substrate using a translucent silicon resin. In order to effectively reflect light of the LEDs, the substrate is formed of epoxy resin into which titanium oxide powders are mixed, for example, and is formed so as to have a white surface.
When the LEDs are blue light-emitting diodes, however, since titanium oxide is easily resolved by blue light, the surface of the substrate deteriorates with time and becomes rough. When the surface of the substrate becomes rough, reflectance of the surface substrate deteriorates, and luminous efficiency of the light-emitting module deteriorates with time.
In order to increase reflectivity of the substrate surface, an approach of providing a metal reflective layer on a surface of a substrate including LEDs mounted thereon has been proposed. In this case, a plurality of silver reflective layers having relatively high reflectivity are provided on the surface of the substrate of each LED.
When blue light of the LEDs is made incident on the substrate surface exposed between the reflective layers, however, unbonded molecules of the epoxy resin are photodegraded and gasified. The gasified organic components react with the silver reflective layers via a gas-permeable silicon resin sealing the LEDs. Thereby, the top surface of the reflective layer is tarnished to a darkish color. This reaction continues until release of the gas ends.
That is, even by providing metal reflective layers on a surface of the substrate, when the portion of the substrate surface that is exposed without being provided with a reflective layer is large, the reflective layer itself may be tarnished with time, and the reflectivity may deteriorate.